This invention relates to a method for making expandable styrene-type polymers having a short cooling time.
The making of expandable particles of styrene-type polymers is well known. It comprises two basic aspects, polymerization of styrene-type monomer and the impregnation of the polymer with a blowing agent. The polymerization reaction is usually conducted either in bulk or in suspension. Where the bulk polymerization method is used to prepare the polymer the impregnation step is usually conducted in a separate reactor. In the case of the suspension polymerization method the impregnation may be carried out either as a step contiguous to the polymerization by introducing the blowing agent to the reactor at a certain point in the conversion of monomer to polymer or at the completion of the polymerization, or as a step completely separate and independent of the polymerization. In the latter case, the bead product of the suspension polymerization is taken from the reactor, washed, and subsequently, the impregnation step is conducted by re-suspending the polymer in water. The latter method has the advantage that out of the total polymer beads obtained by suspension polymerization, one can select only those beads having a particle size suitable for expandable styrene-type polymer usage and use the rest for other purposes.
The impregnated particles are usually pre-expanded and aged before introduction into the mold to stabilize the bead structure and control the final density of the mold article. The molding step is conducted by filling the mold cavity with the pre-expanded polymer particles and heating them above the softening point usually by use of superheated steam. The minimum time period, after the introduction of steam into the mold, that the article must remain in the mold before removal is generally called the "cooling time". The cooling time is a very substantial portion of the molding cycle and hence the reduction thereof has an important commercial significance for enhancing production efficiency. Many attempts have been made in the past to reduce the cooling time for these products, for instance:
U.S. Pat. No. 3,468,820 (1969) discloses a method of suspension-polymerizing styrene in the presence of a blowing agent and 0.001 to 1% of a water-insoluble high molecular weight polymer of a vinyl ester, vinyl ether, acrylic ester, or methacrylic ester which will dissolve in the monomer but is not homogeneously miscible with the polymer obtained. The patent discloses that the resultant polymer has fine and uniform cells and also a relatively short cooling time. PA0 U.S. Pat. No. 3,480,570 (1969) discloses a method of coating expandable polystyrene beads with 0.02-0.3% by weight of sorbitan fatty acid ester containing 12-18 carbon atoms in the fatty acid moiety and having an HLB (hydrophilic-lipophilic balance) number of 1.5-9. The patent discloses that the cooling time is reduced typically to 50-80% of the "normal" value. PA0 British Pat. No. 1,174,749 (1969) discloses an aqueous suspension polymerization method for preparing expandable polystyrene beads having a reduced cooling time which method comprises impregnating the polymer beads with a blowing agent in the presence of a polyoxyethylene compound after the beads have reached the bead identity point. The polyoxyethylene compound may be a polyoxyethylene monoester of a fatty acid, a polyoxyethylene sorbitan monoester of a fatty acid or a polyoxyethylene monoether of a fatty alcohol, having an HLB number of at least 15. PA0 U.S. Pat. No. 3,991,020 (1969) discloses an expandable polystyrene composition wherein 0.005-1.0% (based on the weight of polystyrene) of a polyether derived from propylene oxide or from propylene oxide and ethylene oxide is homogeneously dispersed on at least the surface of the expandable particles. The admixture of polyethers and polystyrene is prepared by mixing the expandable polystyrene particles with the additive, or a portion of the additive may be incorporated in the polystyrene during polymerization and the remaining portion added to the subsequent mixture. This patent also discloses a process for preparing expandable polystyrene wherein the monomer is polymerized in aqueous suspension, a blowing agent is admixed with the polymer formed during and/or at the end of the polymerization, and the polymer containing the expanding agent is separated from the suspension medium and subsequently mixed with 0.005-1.0% by weight of polyethers derived from propylene oxide or from propylene and ethylene oxides. The main advantages disclosed in this patent are that the expandable beads have an anti-lumping property during the pre-expansion and a short cooling time. PA0 U.S. Pat. No. 4,042,541 (1977) discloses expandable polystyrene beads having a reduced cooling time containing, uniformly dispersed within the beads, from about 100 to 300 ppm of stearic acid and from about 40 to 120 ppm of calcium stearate. PA0 U.S. Pat. No. 4,174,427 (1979) discloses a method for making expandable polystyrene beads suitable for making drinking cups and other impervious containers wherein the polymerization of styrene is conducted in the presence of polyvinyl pyrrolidone, and, after impregnation, the beads are washed in an aqueous solution of a hydrophilic alkoxylated nonionic surfactant, which is solid at room temperature and has 70-90 weight percent of the polymer chain accounted for by oxyethylene units, for about 3 to 10 hours at a temperature of 40.degree.-55.degree. C. to remove contaminated grafted polyvinyl pyrrolidone from the surface of said beads. The primary purpose of this patented process is to obtain polystyrene beads which are expandable into impervious containers and which are free from mold-filling and static-electricity problems.
We have developed a method for making styrene-type expandable polymer beads having a short cooling time wherein the impregnation is carried out in an aqueous suspending medium comprising about 0.005-0.3%, based on the weight of water, of a nonionic surfactant which is a copolymer of propylene oxide and ethylene oxide having an HLB number of 8-18 and an average molecular weight of at least 1,000, and the resultant expandable beads are subsequently washed with water and dried. We have discovered that the use of the particular type of propylene oxide-ethylene oxide copolymer as a surfactant in the impregnation step affords polymer beads the cells of which after pre-expansion are non-uniform; namely, the beads have smaller cells toward the outer peripheral portion and larger cells toward the core. The pre-expanded polymer beads having such "edge-zone" cell structure seem to be more advantageous than those having uniform cell size distribution for the purpose of reducing the cooling time of the final molding process.